1. Field of the Invention
This invention relates generally to an apparatus for concealing an error in a reproduced or received digital video signal.
2. Description of the Prior Art
In recording and reproducing a digital video signal by a rotary head type video tape recorder (VTR), random errors may result from head noise, tape noise or amplifier noise or a burst error may be caused by a signal dropout. It is a recognized advantage of digital signal processing that erroneous data can be mathematically corrected by the inclusion in the recorded or transmitted data of redundant bits. For example, a well known scheme for correcting digital data involves dividing the latter into blocks, each of which is recorded or transmitted along with a cyclic redundancy check (CRC) code and horizontal and vertical parity data so that, upon reproduction or reception, an error in any such block can be detected and then corrected on the basis of the respective parity data. However, the addition to the recorded information data of the redundant bits representing the CRC code and the parity data for the purpose of protecting the information data from error necessarily increases the recording bit rate which is limited by the necessity of minimizing the consumption of tape. Therefore, even if the code arrangement of the digital video signal is designed to be capable of error correction, the extent of the error may sometimes exceed the error correcting ability which is limited by the acceptable redundancy.
It has further been proposed to conceal an error in a transmitted or recorded video signal so that such error will not be noticed in the displayed picture. One error concealing method that has already been proposed involves interpolation of the erroneous data with data of the immediately preceding line of the same field, and such method relies on the strong correlation of a television picture in the vertical direction. Another conventional error concealing method involves replacement of the erroneous data with a mean value of data from the lines immediately preceding and following the line containing the error.
Each of the above error concealing methods obtains the signal which is to be interpolated or substituted for the erroneous data from the data of the same field. However, since the television picture is formed by interlaced scanning, it will be appreciated that adjacent lines in the same field are spaced apart by a distance that is twice the distance between adjacent lines in the pictorial representation of the complete frame made up of two interlaced fields. Therefore, the data in immediately adjacent lines of such pictorial representation of the complete frame and which occur in contiguous fields of the video signal have an even higher correlation therebetween.
In view of the foregoing, it has been proposed by the assignee of this application to effect error concealment by replacing error-containing data in a line of one field with corresponding data in a so-called interpolated line of the next previous field which, in the pictorial representation of the complete frame, is positioned immediately adjacent the error-containing line so that the data used for concealing an error will bear a closer resemblance to the original or correct data which it replaces. However, in the case of color video signals, the phase of the chrominance component, that is, of the color subcarrier of the color information, may not be the same at corresponding locations in immediately adjacent lines of the pictorial representation of a complete frame. In other words, although the color information is the same at corresponding locations in such immediately adjacent lines, the polarities thereof may be relatively inverted. For example, in the case of an NTSC color video signal, if error-containing data in a line of one field is replaced with corresponding data in the line of the next previous field which, in the pictorial representation of a complete frame, is positioned immediately above the error-containing line, the color information in the interpolated line immediately above the error-containing line will be of inverted polarity in respect to the color information in the error-containing line and cannot be used for concealing the latter. Similarly, in the case of a
color video signal, if error-containing data in a line of one field is replaced with corresponding data in the line of the next previous field which, in the pictorial representation of the complete frame, is positioned immediately below the error containing line, the color information of the so-called interpolated line will be the same as, but of inverted polarity in respect to the corresponding color information in the error-containing line. On the other hand, if the interpolated line selected to replace an error-containing line of a PAL color video signal is the line which is immediately above the error-containing line in the pictorial representation of a complete frame, then the color information of each sampling point of the interpolated line will not even correspond to the color information at the corresponding point in the error-containing line.